There are electronic key systems for a specific vehicle as disclosed in Japanese Laid-Open Patent Publication No. 2001-349110 and Japanese Laid-Open Patent Publication No. 2001-349117. In the electronic key systems for a vehicle in Japanese Laid-Open Patent Publication No. 2001-349110 and Japanese Laid-Open Patent Publication No. 2001-349117, an actuating unit (switch) is installed on the door handle or on the trunk lid of the specific vehicle, and when an user operates (actuates) the actuating unit, communication with the electronic key is started and an ID transmitted from the electronic key is verified against an ID registered in the control unit. When it is proved that the result showing that the IDs are identical is obtained, the door lock or the like is released.
In the related art, there is also proposed an electronic key system in which a transmitting antenna is provided on each door of a four-wheeled vehicle, and only the door towards which a mobile machine approaches can be unlocked independently of the other doors of the vehicle (For example, Japanese Laid-Open Patent Publication No. 10-317754).
Furthermore, an electronic key system for a motorcycle has been proposed in which communication between a transmitter-receiver that a user carries and an immobilizer, which has been installed on a specific vehicle can be started by inserting a key into a key cylinder installed on the specific vehicle (For example, Japanese Laid-Open Patent Publication No. 2001-12123).
When such electronic key system is adapted to be used on a motorcycle, an actuating unit, corresponding to the actuating unit (switch) which has been installed on the door handle of the four-wheeled vehicle, must be mounted somewhere on the motorcycle. In particular, when it is applied to a motorcycle having a storage space for a helmet under an openable and closable seat, considering an actuating operation or an action of taking in and out the user's helmet or the like, it is desirable to install the actuating unit at a position the user can naturally access such as on a seat.